This is from an article on BBC back in 2007: "Researchers from Mid Sweden University have constructed an interactive paper billboard that emits recorded sound in response to a user's touch. The prototype display uses conductive inks, which are sensitive to pressure, and printed speakers.

The key to the billboard's capabilities is a layer of digital paper that is embedded with electronics. This is printed with conductive inks, which, when applied with pressure, relay information to a micro-computer that contains recorded audio files. Sound then streams out from printed speakers, which are formed from more layers of conductive inks that sit over an empty cavity to form a diaphragm."